CHANGES IN UTERINE SECRETION OF PROSTAGLANDIN-F2-ALPHA IN RESPONSE TOOXYTOCIN DURING THE ESTROUS-CYCLE, EARLY-PREGNANCY, AND ESTROGEN-INDUCED PSEUDOPREGNANCY IN SWINE

Thirty-one sows were used in an experiment designed to determine wheth er the ability of the porcine uterus to release prostaglandin (PG) F-2 alpha in response to oxytocin was suppressed in pregnancy and pseudop regnancy. Sows were assigned to one of three treatment groups: nonbred (nonpregnant) controls (n = 8), pseudopregnant (5 mg estradiol benzoa te, i.m., daily on Days 11-15 postestrus; n = 8), or bred (bred once d aily throughout the estrous period; n = 15). Jugular venous blood samp les were collected daily for quantification of progesterone. Pregnancy was determined by uterine examination at slaughter 51-72 days postmat ing. On the basis of progesterone and embryo recovery, bred sows were classified into three subgroups: confirmed pregnant (n = 4), suspected pregnant based on delayed luteal regression (n = 5), or bred/not preg nant (n = 6). All sows received an injection of oxytocin (30 IU, i.v.) on Days 12, 15, and 18 postestrus. Jugular venous blood samples were collected from 60 min prior to through 120 min after injection of oxyt ocin for quantification of 13,14-dihydro-15-keto-PGF(2 alpha) (PGFM). Magnitude of response above baseline and area under the PGFM response curve (AUG) were calculated for each sow on each day and compared amon g treatment groups by analysis of variance, Responses in pregnant and suspected-pregnant sows were not different on any day examined; theref ore the two groups were combined (n = 9) and considered pregnant for a ll subsequent analyses. Responses in the nonpregnant and bred/not preg nant sows were pooled and compared to the responses in the pregnant an d pseudopregnant sows. Magnitudes of response were similar between the se pooled groups on Day 12 (p > 0.5), but were less in pregnant and ps eudopregnant sows on Days 15 and 18 (p < 0.01). When nonpregnant and b red/not pregnant sows were compared to each other, the magnitudes of t he response were similar on Days 12, 15, and 18 (p > 0.3 on each day). In contrast, when pregnant and pseudopregnant sows were compared, pse udopregnant sows had a lower magnitude of response that was consistent across all 3 days (p < 0.02). Similar relationships were apparent for the AUC. These results demonstrate that uterine secretory responsiven ess to oxytocin is suppressed during early pregnancy and that this eff ect may be mediated through estrogen secreted by conceptuses.